Every time you inhale smoke from a cigarette, you kill some of the alveoli, or the air sacks in your lungs.
These air sacks are where the oxygen that you breathe in is transferred into your blood.
The alveoli will not grow back.
So if you destroy them, you permanently have destroyed part of your lungs.
Smoking paralyzes the cilia that line your lungs.
Cilia are little hair like structures that move back and forth to sweep particles out of your lungs. When you smoke, the cilia can not move and can not do their job.
So dust, pollen, and other things that you inhale they sit in your lungs and build up.
Also, there are a lot of particles in smoke that get into your lungs. Since your cilia are paralyzed because of the smoke and can not clean them out, the particles sit in your lungs and form tar.